SOAP Version 1.2 Part 0: Primer (Second Edition) is a non-normative document intended to
provide an easily understandable tutorial on the features of SOAP Version
1.2. In particular, it describes the features through various usage
scenarios, and is intended to complement the normative text contained in Part 1 and Part 2 of the
SOAP 1.2 specifications. This second edition includes additional material on
the SOAP Message Transmission Optimization Mechanism (MTOM), the XML-binary
Optimized Packaging (XOP) and the Resource Representation SOAP Header Block
(RRSHB) specifications.

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

Please report errors in this document to the public mailing list xmlp-comments@w3.org
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1. Introduction

SOAP Version 1.2 Part 0: Primer (Second Edition) is a non-normative document intended to
provide an easily understandable tutorial on the features of the SOAP Version
1.2 specifications. Its purpose is to help a technically competent person
understand how SOAP may be used, by describing representative SOAP message
structures and message exchange patterns.

It is expected that the reader has some familiarity with the basic syntax
of XML, including the use of XML namespaces and infosets, and Web concepts
such as URIs and HTTP. It is intended primarily for users of SOAP, such as
application designers, rather than implementors of the SOAP specifications,
although the latter may derive some benefit. This primer aims at highlighting
the essential features of SOAP Version 1.2, not at completeness in describing
every nuance or edge case. Therefore, there is no substitute for the main
specifications to obtain a fuller understanding of SOAP. To that end, this
primer provides extensive links to the main specifications wherever new
concepts are introduced or used.

[SOAP Part1] defines the SOAP envelope, which is a
construct that defines an overall framework for representing the contents of
a SOAP message, identifying who should deal with all or part of it, and
whether handling such parts are optional or mandatory. It also defines a
protocol binding framework, which describes how the specification for a
binding of SOAP onto another underlying protocol may be written.

[SOAP Part2] defines a data model for SOAP, a
particular encoding scheme for data types which may be used for conveying
remote procedure calls (RPC), as well as one concrete realization of the
underlying protocol binding framework defined in [SOAP
Part1]. This binding allows the exchange of SOAP messages either as
payload of a HTTP POST request and response, or as a SOAP message in the
response to a HTTP GET.

[MTOM] describes an abstract feature for optimizing
the wire format of a SOAP message for certain type of content, as well as a
concrete implementation of it realized in an HTTP binding, while still
maintaining the modeling of a SOAP message as a single XML Infoset.

[XOP]defines a convention for serializing more
efficiently an XML Infoset that has binary content.
[MTOM] makes use of the
[XOP] format for optimizing the transmission of SOAP
messages.

[ResRep] specifies a SOAP header block which carries a
representation of a Web resource, which is needed for processing a SOAP
message but which a receiver would prefer not to or cannot obtain by
dereferencing the URI for the resource carried within the message.

This document (the primer) is not normative, which means that it does not
provide the definitive specification of SOAP Version 1.2 or the other
specifications cited above. The examples provided here are intended to
complement the formal specifications, and in any question of interpretation
the formal specifications naturally take precedence. The examples shown here
provide a subset of the uses expected for SOAP. In actual usage scenarios,
SOAP will most likely be a part of an overall solution, and there will no
doubt be other application-specific requirements which are not captured in
these examples.

1.1 Overview

SOAP Version 1.2 provides the definition of the XML-based information
which can be used for exchanging structured and typed information between
peers in a decentralized, distributed environment. [SOAP
Part1] explains that a SOAP message is formally specified as an XML
Information Set [XML Infoset] (henceforth often simply
infoset), which provides an abstract description of its contents. Infosets
can have different on-the-wire representations (aka serializations), one
common example of which is as an XML 1.0 [XML 1.0]
document. However, other serializations are also possible, and [MTOM] using the [XOP] format offers
one mechanism for doing so for the cases where there is a need to optimize
the processing and size of the transmitted message.

SOAP is fundamentally a stateless, one-way message exchange paradigm, but
applications can create more complex interaction patterns (e.g.,
request/response, request/multiple responses, etc.) by combining such one-way
exchanges with features provided by an underlying protocol and/or
application-specific information. SOAP is silent on the semantics of any
application-specific data it conveys, as it is on issues such as the routing
of SOAP messages, reliable data transfer, firewall traversal, etc. However,
SOAP provides the framework by which application-specific information may be
conveyed in an extensible manner. Also, SOAP provides a full description of
the required actions taken by a SOAP node on receiving a SOAP message.

Section 2 of this document provides an introduction
to the basic features of SOAP starting with the simplest usage scenarios,
namely a one-way SOAP message, followed by various request-response type
exchanges, including RPCs. Fault situations are also described.

Section 3 provides an overview of the SOAP processing
model, which describes the rules for initial construction of a message, rules
by which messages are processed when received at an intermediary or ultimate
destination, and rules by which portions of the message can be inserted,
deleted or modified by the actions of an intermediary.

Section 4 of this document describes the ways in
which SOAP messages may be transported to realize various usage scenarios. It
describes the SOAP HTTP binding specified in [SOAP
Part2], as well as an example of how SOAP messages may be conveyed in
email messages. As a part of the HTTP binding, it introduces two message
exchange patterns which are available to an application, one of which uses
the HTTP POST method, while the other uses HTTP GET. Examples are also
provided on how RPCs, in particular those that represent "safe" information
retrieval, may be represented in SOAP message exchanges in a manner that is
compatible with the architectural principles of the World Wide Web .

Section 5 of this document provides a treatment of
various aspects of SOAP that can be used in more complex usage scenarios.
These include the extensibility mechanism offered through the use of header
elements, which may be targeted at specific intermediate SOAP nodes to
provide value-added services to communicating applications, using various
encoding schemes to serialize application-specific data in SOAP messages, and
the means to provide a more optimized serialization of a SOAP message under
certain circumstances.

For ease of reference, terms and concepts used in this primer are
hyper-linked to their definition in the main specifications.

1.2 Notational Conventions

Throughout this primer, sample SOAP envelopes and messages are shown as
[XML 1.0] documents. [SOAP Part1]
explains that a SOAP message is formally specified as an [XML
InfoSet], which is an abstract description of its contents. The
distinction between the SOAP message infosets and their representation as XML
documents is unlikely to be of interest to those using this primer as an
introduction to SOAP; those who do care (typically those who port SOAP to new
protocol bindings where the messages may have alternative representations)
should understand these examples as referring to the corresponding XML
infosets. Further elaboration of this point is provided in Section 4 of this document.

The namespace prefixes "xs" and "xsi" used in the prose sections of this
document are associated with the namespace names
"http://www.w3.org/2001/XMLSchema" and
"http://www.w3.org/2001/XMLSchema-instance" respectively, both of which are
defined in the XML Schema specifications [XML Schema
Part1], [XML Schema Part2].

Note that the choice of any other namespace prefix is arbitrary and not
semantically significant.

Namespace URIs of the general form "http://example.org/..." and
"http://example.com/..." represent an application-dependent or
context-dependent URI [RFC 3986].

2. Basic Usage Scenarios

A
SOAP message is fundamentally a one-way transmission between SOAP
nodes, from a
SOAP sender to a
SOAP receiver, but SOAP messages are expected to be combined by
applications to implement more complex interaction patterns ranging from
request/response to multiple, back-and-forth "conversational" exchanges.

The primer starts by exposing the structure of a SOAP message and its
exchange in some simple usage scenarios based on a travel reservation
application. Various aspects of this application scenario will be used
throughout the primer. In this scenario, the travel reservation application
for an employee of a company negotiates a travel reservation with a travel
booking service for a planned trip. The information exchanged between the
travel reservation application and the travel service application is in the
form of SOAP messages.

The ultimate recipient of a SOAP message sent from the travel reservation
application is the travel service application, but it is possible that the
SOAP message may be "routed" through one or more
SOAP intermediaries which act in some way on the message. Some simple
examples of such SOAP intermediaries might be ones that log, audit or,
possibly, amend each travel request. Examples, and a more detailed discussion
of the behavior and role of SOAP intermediaries, is postponed to section 5.1.

Section 2.1 describes a travel reservation request
expressed as a SOAP message, which offers the opportunity to describe the
various "parts" of a SOAP message.

Section 2.2.1 continues the same scenario to show a
response from the travel service in the form of another SOAP message, which
forms a part of a conversational message exchange as the various choices
meeting the constraints of the travel request are negotiated.

Section 2.2.2 assumes that the various parameters of
the travel reservation have been accepted by the traveller, and an exchange -
modelled as a remote procedure call (RPC) - between the travel reservation
and the travel service applications confirms the payment for the
reservation.

Sample SOAP message for a travel reservation containing header blocks and a
body

The SOAP message in Example 1 contains two
SOAP-specific sub-elements within the overall env:Envelope,
namely an env:Header
and an env:Body.
The contents of these elements are application defined and not a part of the
SOAP specifications, although the latter do have something to say about how
such elements must be handled.

A
SOAP header element is optional, but it has been included in the example
to explain certain features of SOAP. A SOAP header is an extension mechanism
that provides a way to pass information in SOAP messages that is not
application payload. Such "control" information includes, for example,
passing directives or contextual information related to the processing of the
message. This allows a SOAP message to be extended in an application-specific
manner. The immediate child elements of the env:Header element
are called
header blocks, and represent a logical grouping of data which, as shown
later, can individually be targeted at SOAP nodes that might be encountered
in the path of a message from a sender to an ultimate receiver.

SOAP headers have been designed in anticipation of various uses for SOAP,
many of which will involve the participation of other SOAP processing nodes -
called
SOAP intermediaries - along a message's path from an initial
SOAP sender to an ultimate
SOAP receiver. This allows SOAP intermediaries to provide value-added
services. Headers, as shown later, may be inspected, inserted, deleted or
forwarded by SOAP nodes encountered along a SOAP
message path. (It should be kept in mind, though, that the SOAP
specifications do not deal with what the contents of header elements are, or
how SOAP messages are routed between nodes, or the manner by which the route
is determined and so forth. These are a part of the overall application, and
could be the subject of other specifications.)

The
SOAP body is the mandatory element within the SOAP env:Envelope,
which implies that this is where the main end-to-end information conveyed in
a SOAP message must be carried.

A pictorial representation of the SOAP message in Example 1 is as follows.

Figure 1: SOAP message structure

In Example 1, the header contains two header
blocks, each of which is defined in its own XML namespace and which represent
some aspect pertaining to the overall processing of the body of the SOAP
message. For this travel reservation application, such "meta" information
pertaining to the overall request is a reservation header block
which provides a reference and time stamp for this instance of a reservation,
and the traveller's identity in the passenger block.

The header blocks reservation and passenger must
be processed by the next SOAP intermediary encountered in the message path
or, if there is no intermediary, by the ultimate recipient of the message.
The fact that it is targeted at the next SOAP node encountered en
route is indicated by the presence of the attribute env:role
with the value "http://www.w3.org/2003/05/soap-envelope/role/next" (hereafter
simply "next"), which is a
role that all SOAP nodes must be willing to play. The presence of an
env:mustUnderstand attribute with value "true" indicates that
the node(s) processing the header must absolutely process these header blocks
in a manner consistent with their specifications, or else not process the
message at all and throw a fault. Note that whenever a header block is
processed, either because it is marked env:mustUnderstand="true"
or for another reason, the block must be processed in accordance with the
specifications for that block. Such header block specifications are
application defined and not a part of SOAP. Section 3
will elaborate further on SOAP message processing based on the values of
these attributes.

The choices of what data is placed in a header block and what goes in the
SOAP body are decisions made at the time of application design. The main
point to keep in mind is that header blocks may be targeted at various nodes
that might be encountered along a message's path from a sender to the
ultimate recipient. Such intermediate SOAP nodes may provide value-added
services based on data in such headers. In Example 1,
the passenger data is placed in a header block to illustrate the use of this
data at a SOAP intermediary to do some additional processing. For example, as
shown later in section 5.1, the outbound message is
altered by the SOAP intermediary by having the travel policies pertaining to
this passenger appended to the message as another header block.

The env:Body
element and its associated child elements, itinerary and
lodging, are intended for exchange of information between the
initial SOAP sender and the SOAP node which assumes the role of the
ultimate SOAP receiver in the message path, which is the travel service
application. Therefore, the env:Body and its contents are
implicitly targeted and are expected to be understood by the ultimate
receiver. The means by which a SOAP node assumes such a role is not defined
by the SOAP specification, and is determined as a part of the overall
application semantics and associated message flow.

Note that a SOAP intermediary may decide to play the role of the ultimate
SOAP receiver for a given message transfer, and thus process the
env:Body. However, even though this sort of a behavior cannot be
prevented, it is not something that should be done lightly as it may pervert
the intentions of the message's sender, and have undesirable side effects
(such as not processing header blocks that might be targeted at
intermediaries further along the message path).

A SOAP message such as that in Example 1 may be
transferred by different underlying protocols and used in a variety of message
exchange patterns. For example, for a Web-based access to a travel
service application, it could be placed in the body of a HTTP POST request.
In another protocol binding, it might be sent in an email message (see section 4.2). Section 4 will
describe how SOAP messages may be conveyed by a variety of underlying
protocols. For the time being, it is assumed that a mechanism exists for
message transfer and the remainder of this section concentrates on the
details of the SOAP messages and their processing.

2.2 SOAP Message Exchange

SOAP Version 1.2 is a simple messaging framework for transferring
information specified in the form of an XML infoset between an initial SOAP
sender and an ultimate SOAP receiver. The more interesting scenarios
typically involve multiple message exchanges between these two nodes. The
simplest such exchange is a request-response pattern. Some early uses of [SOAP 1.1] emphasized the use of this pattern as means for
conveying remote procedure calls (RPC), but it is important to note that not
all SOAP request-response exchanges can or need to be modelled as RPCs. The
latter is used when there is a need to model a certain programmatic behavior,
with the exchanged messages conforming to a pre-defined description of the
remote call and its return.

A much larger set of usage scenarios than that covered by the
request-response pattern can be modeled simply as XML-based content exchanged
in SOAP messages to form a back-and-forth "conversation", where the semantics
are at the level of the sending and receiving applications. Section 2.2.1 covers the case of XML-based content exchanged
in SOAP messages between the travel reservation application and the travel
service application in a conversational pattern, while section 2.2.2 provides an example of an exchange modeled as
an RPC.

2.2.1 Conversational Message Exchanges

Continuing with the travel request scenario, Example
2 shows a SOAP message returned from the travel service in response to
the reservation request message in Example 1. This
response seeks to refine some information in the request, namely the choice
of airports in the departing city.

As described earlier, the env:Body contains the primary
content of the message, which in this example includes a list of the various
alternatives for the airport, conforming to a schema definition in the XML
namespace http://travelcompany.example.org/reservation/travel. In this
example, the header blocks from Example 1 are returned
(with some sub-element values altered) in the response. This could allow
message correlation at the SOAP level, but such headers are very likely to
also have other application-specific uses.

The message exchanges in Examples 1 and 2 are
cases where XML-based contents conforming to some application-defined schema
are exchanged via SOAP messages. Once again, a discussion of the means by
which such messages are transferred is deferred to section 4.

It is easy enough to see how such exchanges can build up to a multiple
back-and-forth "conversational" message exchange pattern. Example 3 shows a SOAP message sent by the travel
reservation application in response to that in Example
2 choosing one from the list of available airports. The header block
reservation with the same value of the reference
sub-element accompanies each message in this conversation, thereby offering a
way, should it be needed, to correlate the messages exchanged between them at
the application level.

Response to the message in Example 2 continuing a
conversational message exchange

2.2.2 Remote Procedure Calls

One of the design goals of SOAP Version 1.2 is to encapsulate remote
procedure call functionality using the extensibility and flexibility of XML.
SOAP Part 2 section 4 has defined a uniform representation for RPC
invocations and responses carried in SOAP messages. This section continues
with the travel reservation scenario to illustrate the use of SOAP messages
to convey remote procedure calls and their return.

To that end, the next example shows the payment for the trip using a
credit card. (It is assumed that the conversational exchanges described in section 2.2.1 have resulted in a confirmed itinerary.) Here,
it is further assumed that the payment happens in the context of an overall
transaction where the credit card is charged only when the travel and the
lodging (not shown in any example, but presumably reserved in a similar
manner) are both confirmed. The travel reservation application provides
credit card information and the successful completion of the different
activities results in the card being charged and a reservation code returned.
This reserve-and-charge interaction between the travel reservation
application and the travel service application is modeled as a SOAP RPC.

To invoke a SOAP RPC, the following information is needed:

The address of the target SOAP node.

The procedure or method name.

The identities and values of any arguments to be passed to the
procedure or method together with any output parameters and return
value.

A clear separation of the arguments
used to identify the Web resource which is the actual target for the RPC,
as contrasted with those that convey data or control information used for
processing the call by the target resource.

The
message exchange pattern which will be employed to convey the RPC,
together with an identification of the so-called "Web Method" (on which
more later) to be used.

Optionally, data which may be carried as a part of
SOAP header blocks.

Such information may be expressed by a variety of means, including formal
Interface Definition Languages (IDL). Note that SOAP does not provide any
IDL, formal or informal. Note also that the above information differs in
subtle ways from information generally needed to invoke other, non-SOAP
RPCs.

Regarding Item 1 above, there is, from a SOAP
perspective, a SOAP node which "contains" or "supports" the target of the
RPC. It is the SOAP node which (appropriately) adopts the role of the
ultimate SOAP receiver. As required by Item 1,
the ultimate recipient can identify the target of the named procedure or
method by looking for its URI. The manner in which the target URI is made
available depends on the underlying protocol binding. One possibility is that
the URI identifying the target is carried in a SOAP header block. Some
protocol bindings, such as the SOAP HTTP binding defined in [SOAP Part2], offer a mechanism for carrying the URI outside
the SOAP message. In general, one of the properties of a protocol binding
specification must be a description of how the target URI is carried as a
part of the binding. Section 4.1 provides some concrete
examples of how the URI is carried in the case of the standardized SOAP
protocol binding to HTTP.

Item 4 and Item 5 above are required to ensure
that RPC applications that employ SOAP can do so in a manner which is
compatible with the architectural principles of the World Wide Web. Section 4.1.3 discusses how the information provided by
items 4 and 5 are utilized.

For the remainder of this section, it is assumed that the RPC conveyed in
a SOAP message as shown in Example 4 is appropriately
targeted and dispatched. The purpose of this section is to highlight the
syntactical aspects of RPC requests and returns carried within a SOAP
message.

SOAP RPC request with a mandatory header and two input (or "in")
parameters

The RPC itself is carried as a child of the env:Body element,
and is modelled as a struct which takes the name of the
procedure or method, in this case chargeReservation. (A
struct is a concept
from the SOAP Data Model defined in [SOAP Part2]
that models a structure or record type that occurs in some common programming
languages.) The design of the RPC in the example (whose formal description
has not been explicitly provided) takes two input (or "in") parameters, the
reservation corresponding to the planned trip identified by the
reservation code, and the creditCard information.
The latter is also a struct, which takes three elements, the
card holder's name, the card number and an
expiration date.

In this example, the env:encodingStyle attribute with the
value http://www.w3.org/2003/05/soap-encoding
shows that the contents of the chargeReservation structure have
been serialized according to the SOAP encoding rules, i.e., the particular
rules defined in SOAP
Part 2 section 3. Even though SOAP specifies this particular encoding
scheme, its use is optional and the specification makes clear that other
encoding schemes may be used for application-specific data within a SOAP
message. It is for this purpose that it provides the
env:encodingStyle attribute to qualify header blocks and body
sub-elements. The choice of the value for this attribute is an
application-specific decision and the ability of a caller and callee to
interoperate is assumed to have been settled "out-of-band". Section 5.2 shows an example of using another encoding
scheme.

As noted in Item 6 above, RPCs may also require
additional information to be carried, which can be important for the
processing of the call in a distributed environment, but which are not a part
of the formal procedure or method description. (Note, however, that providing
such additional contextual information is not specific to RPCs, but may be
required in general for the processing of any distributed application.) In
the example, the RPC is carried out in the context of an overall transaction
which involves several activities which must all complete successfully before
the RPC returns successfully. Example 4 shows how a
header block transaction directed at the ultimate recipient
(implied by the absence of the env:role attribute) is used to
carry such information. (The value "5" is some transaction identifier set by
and meaningful to the application. No further elaboration of the
application-specific semantics of this header are provided here, as it is not
germane to the discussion of the syntactical aspects of SOAP RPC
messages.)

Let us assume that the RPC in the charging example has been designed to
have the procedure description which indicates that there are two output (or
"out") parameters, one providing the reference code for the reservation and
the other a URL where the details of the reservation may be viewed. The RPC
response is returned in the env:Body element of a SOAP message,
which is modeled as a struct taking the procedure name
chargeReservation and, as a convention, the word "Response"
appended. The two output (or "out") parameters accompanying the response are
the alphanumeric code identifying the reservation in question,
and a URI for the location, viewAt, from where the reservation
may be retrieved.

This is shown in Example 5a, where the header
again identifies the transaction within which this RPC is performed.

RPC response with two output (or "out") parameters for the call shown in Example 4

RPCs often have descriptions where a particular output parameter is
distinguished, the so-called "return" value. The SOAP
RPC convention offers a way to distinguish this "return" value from the
other output parameters in the procedure description. To show this, the
charging example is modified to have an RPC description that is almost the
same as that for Example 5a, i.e, with the same two
"out" parameters, but in addition it also has a "return" value, which is an
enumeration with potential values of "confirmed" and "pending". The RPC
response conforming to this description is shown in Example 5b, where the SOAP header, as before,
identifies the transaction within which this RPC is performed.

RPC response with a "return" value and two "out" parameters for the call
shown in Example 4

In Example 5b, the return value is identified by
the element rpc:result, and contains the XML Qualified Name (of
type xs:QName) of another element within the struct
which is m:status. This, in turn, contains the actual return
value, "confirmed". This technique allows the actual return value to be
strongly typed according to some schema. If the rpc:result
element is absent, as is the case in Example 5a, the
return value is not present or is of the type void.

While, in principle, using SOAP for RPC is independent of the decision to
use a particular means for transferring the RPC call and its return, certain
protocol bindings that support the SOAP
Request-Response message exchange pattern may be more naturally suited
for such purposes. A protocol binding supporting this message exchange
pattern can provide the correlation between a request and a response. Of
course, the designer of an RPC-based application could choose to put a
correlation ID relating a call and its return in a SOAP header, thereby
making the RPC independent of any underlying transfer mechanism. In any case,
application designers have to be aware of all the characteristics of the
particular protocols chosen for transferring SOAP RPCs, such as latency,
synchrony, etc.

In the commonly used case, standardized in SOAP
Part 2 section 7, of using HTTP as the underlying transfer protocol, an
RPC invocation maps naturally to the HTTP request and an RPC response maps to
the HTTP response. Section 4.1 provides examples of
carrying RPCs using the HTTP binding.

However, it is worth keeping in mind that even though most examples of
SOAP for RPC use the HTTP protocol binding, it is not limited to that means
alone.

2.3 Fault Scenarios

SOAP provides a model for handling situations when faults arise in the
processing of a message. SOAP distinguishes between the conditions that
result in a fault, and the ability to signal that fault to the originator of
the faulty message or another node. The ability to signal the fault depends
on the message transfer mechanism used, and one aspect of the binding
specification of SOAP onto an underlying protocol is to specify how faults
are signalled, if at all. The remainder of this section assumes that a
transfer mechanism is available for signalling faults generated while
processing received messages, and concentrates on the structure of the SOAP
fault message.

The SOAP env:Body element has another distinguished role in
that it is the place where such fault information is placed. The SOAP fault
model (see
SOAP Part 1, section 2.6) requires that all SOAP-specific and
application-specific faults be reported using a single distinguished
element,
env:Fault, carried within the env:Body element. The
env:Fault element contains two mandatory sub-elements, env:Code
and env:Reason,
and (optionally) application-specific information in the env:Detail
sub-element. Another optional sub-element,
env:Node, identifies via a URI the SOAP node which generated the
fault, its absence implying that it was the ultimate recipient of the message
which did so. There is yet another optional sub-element, env:Role,
which identifies the role being played by the node which generated the
fault.

In Example 6a, the top-level
env:Value uses a standardized XML Qualified Name (of type
xs:QName) to identify that it is an env:Sender
fault, which indicates that it is related to some syntactical error or
inappropriate information in the message. (When a env:Sender
fault is received by the sender, it is expected that some corrective action
is taken before a similar message is sent again.) The
env:Subcode element is optional, and, if present, as it is in
this example, qualifies the parent value further. In Example 6a, the env:Subcode denotes that an
RPC specific fault, rpc:BadArguments, defined in SOAP
Part 2 section 4.4, is the cause of the failure to process the
request.

The structure of the env:Subcode
element has been chosen to be hierarchical - each child
env:Subcode element has a mandatory env:Value
and an optional env:Subcode sub-element - to allow
application-specific codes to be carried. This hierarchical structure of the
env:Code element allows for an uniform mechanism for conveying
multiple level of fault codes. The top-level env:Value
is a base fault that is specified in the SOAP Version 1.2 specifications (see
SOAP
Part 1 section 5.4.6) and must be understood by all SOAP nodes. Nested
env:Values are application-specific, and represent further
elaboration or refinement of the base fault from an application perspective.
Some of these values may well be standardized, such as the RPC codes
standardized in SOAP 1.2 (see SOAP
Part 2 section 4.4), or in some other standards that use SOAP as an
encapsulation protocol. The only requirement for defining such
application-specific subcode values is that they be namespace qualified using
any namespace other than the SOAP env namespace which
defines the main classifications for SOAP faults. There is no requirement
from a SOAP perspective that applications need to understand, or even look at
all levels of the subcode values.

The env:Reason
sub-element is not meant for algorithmic processing, but rather for human
understanding; so, even though this is a mandatory item, the chosen value
need not be standardized. Therefore all that is required is that it
reasonably accurately describe the fault situation. It must have one or more
env:Text
sub-elements, each with a unique xml:lang attribute, which
allows applications to make the fault reason available in multiple languages.
(Applications could negotiate the language of the fault text using a
mechanism built using SOAP headers; however this is outside the scope of the
SOAP specifications.)

The absence of a env:Node sub-element within
env:Fault in Example 6a implies that it
is generated by the ultimate receiver of the call. The contents of
env:Detail, as shown in the example, are
application-specific.

During the processing of a SOAP message, a fault may also be generated if
a mandatory header element is not understood or the information contained in
it cannot be processed. Errors in processing a header block are also
signalled using a env:Fault element within the
env:Body, but with a particular distinguished header block,
env:NotUnderstood,
that identifies the offending header block.

Example 6b shows an example of a response to the
RPC in Example 4 indicating a failure to process the
t:transaction header block. Note the presence of the env:MustUnderstand
fault code in the env:Body, and the identification of the header
not understood using an (unqualified) attribute, qname, in the
special (empty) header block env:NotUnderstood.

If there were several mandatory header blocks that were not understood,
then each could be identified by its qname attribute in a series
of such env:NotUnderstood header blocks.

3. SOAP Processing Model

Having established the various syntactical aspects of a SOAP message as
well as some basic message exchange patterns, this section provides a general
overview of the SOAP processing model (specified in
SOAP Part 1, section 2). The SOAP processing model describes the actions
taken by a SOAP node on receiving a SOAP message.

Example 7a shows a SOAP message with several
header blocks (with their contents omitted for brevity). Variations of this
will be used in the remainder of this section to illustrate various aspects
of the processing model.

The SOAP processing model describes the (logical) actions taken by a SOAP
node on receiving a SOAP message. There is a requirement for the node to
analyze those parts of a message that are SOAP-specific, namely those
elements in the SOAP "env" namespace. Such
elements are the envelope itself, the header element and the body element. A
first step is, of course, the overall check that the SOAP message is
syntactically correct. That is, it conforms to the SOAP XML infoset subject
to the restrictions on the use of certain XML constructs - Processing
Instructions and Document Type Definitions - as defined in SOAP
Part 1, section 5.

3.1 The "role" Attribute

Further processing of header blocks and the body depend on the
role(s) assumed by the SOAP node for the processing of a given message.
SOAP defines the (optional) env:role attribute - syntactically,
xs:anyURI - that may be present in a header block, which
identifies the role played by the intended target of that header block. A
SOAP node is required to process a header block if it assumes the role
identified by the value of the URI. How a SOAP node assumes a particular role
is not a part of the SOAP specifications.

In Example 7a, the header block
oneBlock is targeted at any SOAP node that plays the
application-defined role defined by the URI http://example.com/Log. For
purposes of illustration, it is assumed that the specification for such a
header block requires that any SOAP node adopting this role log the entire
message.

Every SOAP node receiving a message with a header block that has a
env:role attribute of "next" must be capable of processing the
contents of the element, as this is a standardized role that every SOAP node
must be willing to assume. A header block thus attributed is one which is
expected to be examined and (possibly) processed by the next SOAP node along
the path of a message, assuming that such a header has not been removed as a
result of processing at some node earlier in the message path.

In Example 7a, the header block
anotherBlock is targeted at the next node in the message path.
In this case, the SOAP message received by the node
playing the application-defined role of "http://example.com/Log", must also
be willing to play the SOAP-defined role of "next". This is also true for the
node which is the ultimate recipient of the message, as it obviously (and
implicitly) also plays the "next" role by virtue of being next in the message
path.

The third header block, aThirdBlock, in Example 7a does not have the env:role
attribute. It is targeted at a SOAP node which assumes the "ultimateReceiver"
role. The "ultimateReceiver" role (which can be explicitly declared or is
implicit if the env:role attribute is absent in a header block)
is played by a SOAP node that assumes the role of the ultimate recipient of a
particular SOAP message. The absence of a env:role attribute in
the aThirdBlock header block means that this header element is
targeted at the SOAP node that assumes the "ultimateReceiver" role.

Note that the env:Body element does not have a
env:role attribute. The body element is always targeted
at the SOAP node that assumes the "ultimateReceiver" role. In that sense, the
body element is just like a header block targeted at the ultimate receiver,
but it has been distinguished to allow for SOAP nodes (typically SOAP
intermediaries) to skip over it if they assume roles other than that of the
ultimate receiver. SOAP does not prescribe any structure for the
env:Body element, except that it recommends that any
sub-elements be XML namespace qualified. Some applications, such as that in
Example 1, may choose to organize the sub-elements of
env:Body in blocks, but this is not of concern to the SOAP
processing model.

The other distinguished role for the env:Body element, as the
container where information on SOAP-specific faults, i.e., failure to process
elements of a SOAP message, is placed has been described previously in section 2.3.

If a header element has the standardized env:role attribute
with value "none", it means that no SOAP node should process the contents,
although a node may need to examine it if the content are data referenced by
another header element that is targeted at the particular SOAP node.

If the env:role attribute has an empty value, i.e.,
env:role="", it means that the relative URI identifying the
role is resolved to the base URI for the SOAP message in question. SOAP
Version 1.2 does not define a base URI for a SOAP message, but defers to the
mechanisms defined in [XMLBase] for deriving the base
URI, which can be used to make any relative URIs absolute. One such
mechanism is for the protocol binding to establish a base URI, possibly by
reference to the encapsulating protocol in which the SOAP message is
embedded for transport. (In fact, when SOAP messages are transported using
HTTP, SOAP Part
2 section 7.1.2 defines the base URI as the Request-URI of the HTTP
request, or the value of the HTTP Content-Location header.)

The following table summarizes the applicable standardized roles that may
be assumed at various SOAP nodes. ("Yes" and "No" means that the
corresponding node does or does not, respectively, play the named role.)

SOAP message showing a variety of header blocks, one of which is mandatory
for processing

After a SOAP node has correctly identified the header blocks (and possibly
the body) targeted at itself using the env:role attribute, the
additional attribute, env:mustUnderstand, in the header elements
determines further processing actions that have to be taken. In order to
ensure that SOAP nodes do not ignore header blocks which are important to the
overall purpose of the application, SOAP header blocks also provide for the
additional optional attribute, env:mustUnderstand, which, if
"true", means that the targeted SOAP node must process the
block according to the specification of that block. Such a block is
colloquially referred to as a mandatory header block. In fact, processing of
the SOAP message must not even start until the node has identified all the
mandatory header blocks targeted at itself, and "understood" them.
Understanding a header means that the node must be prepared to do whatever is
described in the specification of that block. (Keep in mind that the
specifications of header blocks are not a part of the SOAP
specifications.)

In Example 7b, the header block
oneBlock is marked with a env:mustUnderstand value
set to "true", which means that it is mandatory to process this block if the
SOAP node plays the role identified by "http://example.com/Log". The other
two header blocks are not so marked, which means that SOAP node at which
these blocks are targeted need not process them. (Presumably the
specifications for these blocks allow for this.)

A env:mustUnderstand value of "true" means that the SOAP node
must process the header with the semantics described in that header's
specification, or else generate a SOAP fault. Processing the header
appropriately may include removing the header from any generated SOAP
message, reinserting the header with the same or altered value, or inserting
a new header. The inability to process a mandatory header requires that all
further processing of the SOAP message cease, and a SOAP fault be generated.
The message is not forwarded any further.

The env:Body element has no env:mustUnderstand
attribute but it must be processed by the ultimate recipient. In Example 7b, the ultimate recipient of the message - the
SOAP node which plays the "ultimateReceiver" role - must process the
env:Body and may process the header block
aThirdBlock. It may also process the header block
anotherBlock, as it is targeted at it (in the role of "next")
but it is not mandatory to do so if the specifications for processing the
blocks do not demand it. (If the specification for anotherBlock
demanded that it must be processed at the next recipient, it would have
required that it be marked with a env:mustUnderstand="true".)

The role(s) a SOAP node plays when processing a SOAP message can be
determined by many factors. The role could be known a priori, or set
by some out-of-band means, or a node can inspect all parts of a received
message to determine which roles it will assume before processing the
message. An interesting case arises when a SOAP node, during the course of
processing a message, decides that there are additional roles that it needs
to adopt. No matter when this determination is made, externally it must
appear as though the processing model has been adhered to. That is, it must
appear as though the role had been known from the start of the processing of
the message. In particular, the external appearance must be that the
env:mustUnderstand checking of any headers with those additional
roles assumed was performed before any processing began. Also, if a SOAP node
assumes such additional roles, it must ensure that it is prepared to do
everything that the specifications for those roles require.

The following table summarizes how the processing actions for a header
block are qualified by the env:mustUnderstand attribute with
respect to a node that has been appropriately targeted (via the
env:role attribute).

Node

intermediary

ultimate receiver

mustUnderstand

"true"

must process

must process

"false"

may process

may process

absent

may process

may process

As a result of processing a SOAP message, a SOAP node may generate a
single SOAP fault if it fails to process a message, or, depending on the
application, generate additional SOAP messages for consumption at other SOAP
nodes. SOAP
Part 1 section 5.4 describes the structure of the fault message while the
SOAP
processing model defines the conditions under which it is generated. As
illustrated previously in section 2.3, a SOAP fault is
a SOAP message with a standardized env:Body sub-element named
env:Fault.

SOAP makes a distinction between generating a fault and ensuring that the
fault is returned to the originator of the message or to another appropriate
node which can benefit from this information. However, whether a generated
fault can be propagated appropriately depends on the underlying protocol
binding chosen for the SOAP message message exchange. The specification does
not define what happens if faults are generated during the propagation of
one-way messages. The only normative underlying protocol binding, which is
the SOAP HTTP binding, offers the HTTP response as a means for reporting a
fault in the incoming SOAP message. (See Section 4
for more details on SOAP protocol bindings.)

3.3 The "relay" Attribute

SOAP Version 1.2 defines another optional attribute for header blocks,
env:relay
of type xs:boolean, which indicates if a header block targeted
at a SOAP intermediary must be relayed if it is not processed.

Note that if a header block is processed, the SOAP processing
rules (see SOAP
Part 1 section 2.7.2) requires that it be removed from the outbound
message. (It may, however, be reinserted, either unchanged or with its
contents altered, if the processing of other header blocks determines that
the header block be retained in the forwarded message.) The default behavior
for an unprocessed header block targeted at a role played by a SOAP
intermediary is that it must be removed before the message is relayed.

The reason for this choice of default is to lean on the side of safety by
ensuring that a SOAP intermediary make no assumptions about the survivability
past itself of a header block targeted at a role it assumes, and representing
some value-added feature, particularly if it chooses not to process the
header block, very likely because it does not "understand" it. That is
because certain header blocks represent hop-by-hop features, and it may not
make sense to unknowingly propagate it end-to-end. As an intermediary may not
be in a position to make this determination, it was thought that it would be
safer if unprocessed header blocks were removed before the message was
relayed.

However, there are instances when an application designer would like to
introduce a new feature, manifested through a SOAP header block, targeted at
any capable intermediary which might be encountered in the SOAP
message path. Such a header block would be available to those intermediaries
that "understood" it, but ignored and relayed onwards by those that did not.
Being a new feature, the processing software for this header block may be
implemented, at least initially, in some but not all SOAP nodes. Marking such
a header block with env:mustUnderstand = "false" is obviously
needed, so that intermediaries that have not implemented the feature do not
generate a fault. To circumvent the default rule of the processing model,
marking a header block with the additional attribute env:relay
with the value "true" allows the intermediary to forward the header block
targeted at itself in the event that it chooses not to process it.

Targeting the header block at the role "next" together with the
env:relay attribute set to "true" can always serve to ensure
that each intermediary has a chance to examine the header, because one of the
anticipated uses of the "next" role is with header blocks that carry
information that are expected to persist along a SOAP message path. Of
course, the application designer can always define a custom role that allows
targetting at specific intermediaries that assume this role. Therefore, there
is no restriction on the use of the env:relay attribute with any
role except of course the roles of "none" and "ultimateReceiver", for which
it is meaningless.

SOAP message showing a variety of header blocks, one of which must be relayed
if unprocessed.

The header block q:anotherBlock, targeted at the "next" node
in the message path, has the additional attribute
env:relay="true". A SOAP node receiving this message may process
this header block if it "understands" it, but if it does so the processing
rules require that this header block be removed before forwarding. However,
if the SOAP node chooses to ignore this header block, which it can because it
is not mandatory to process it, as indicated by the absence of the
env:mustUnderstand attribute, then it must forward it.

Processing the header block p:oneBlock is mandatory and the
SOAP processing rules require that it not be relayed, unless the processing
of some other header block requires that it be present in the outbound
message. The header block r:aThirdBlock does not have an
env:relay attribute, which is equivalent to having it with the
value of env:relay = "false". Hence, this header is not
forwarded if it is not processed.

SOAP
1.2 Part 1 Table 3 summarizes the conditions which determine when a SOAP
intermediary assuming a given role is allowed to forward unprocessed header
blocks.

4. Using Various Protocol Bindings

SOAP messages may be exchanged using a variety of "underlying" protocols,
including other application layer protocols. The specification of how SOAP
messages may be passed from one SOAP node to another using an underlying
protocol is called a SOAP
binding. [SOAP Part1] defines a SOAP message in the
form of an [XML Infoset], i.e., in terms of element and
attribute information items of an abstract "document" called the
env:Envelope (see SOAP
Part 1, section 5). Any SOAP env:Envelope infoset
representation will be made concrete through a protocol binding, whose task,
among other things, it is to provide a serialized representation of the
infoset that can be conveyed to the next SOAP node in the message path in a
manner such that the original infoset can be reconstructed without loss of
information.

In typical examples of SOAP messages, and certainly in all the examples in
this primer, the serialization shown is that of a well-formed [XML 1.0] document. However, there may be
other protocol bindings - for example a protocol binding between two SOAP
nodes over a limited bandwidth interface - where an alternative, compressed
serialization of the same infoset may be chosen. Another binding, chosen for
a different purpose, may provide a serialization which is an encrypted
structure representing the same infoset. The [MTOM]
specification provides a SOAP binding to HTTP that allows for an optimized
serialization of the SOAP message infoset under certain circumstances. A more
detailed discussion of this binding is deferred to Section
5.3.

In addition to providing a concrete realization of a SOAP infoset between
adjacent SOAP nodes along a SOAP message path, a protocol binding provides
the mechanisms to support features
that are needed by a SOAP application. A feature is a specification of a
certain functionality required in the interactions between two SOAP nodes,
which may be provided by a binding. A feature description is identified by a
URI, so that all applications referencing it are assured of the same
semantics. Features are qualified by properties,
which provide additional information that help in the implementation of the
feature. For example, a typical usage scenario might require many concurrent
request-response exchanges between adjacent SOAP nodes, in which case the
feature that is required is the ability to correlate a request with a
response. The abstract property associated with this feature is a
"correlation ID". Other examples includes "an encrypted channel" feature, or
a "reliable delivery channel" feature, or a particular SOAP
message exchange pattern feature. In particular, the [MTOM] specification defines an Abstract
SOAP Transmission Optimization feature which may be used by SOAP bindings
to optimize the serialization of selected element information items of a SOAP
message infoset. (See section 5.3.1 for details).

A SOAP binding specification (see SOAP
Part 1 section 4) describes, among other things, which (if any) features
it provides. Some features may be provided natively by the underlying
protocol. If the feature is not available through the binding, it may be
implemented within the SOAP envelope, using SOAP header blocks. The
specification of a feature implemented using SOAP header blocks is called a
SOAP
module.

For example, if SOAP message exchanges were being transported directly
over a datagram protocol like UDP, obviously the message correlation feature
mentioned earlier would have to be provided by other means, either directly
by the application or more likely as a part of the SOAP infosets being
exchanged. In the latter case, the message correlation feature has a
binding-specific expression within the SOAP envelope, i.e., as a SOAP header
block, defined in a "Request-Response Correlation" module identified by a
URI. However, if the SOAP infosets were being exchanged using an underlying
protocol that was itself request/response, the application could implicitly
"inherit" this feature provided by the binding, and no further support need
be provided at the application or the SOAP level. (In fact, the HTTP binding
for SOAP takes advantage of just this feature of HTTP.) The Abstract SOAP
Transmission Optimization feature defined in [MTOM] is similarly implemented as a part of an augmented
SOAP HTTP binding, by serializing particular nodes of a SOAP message infoset
in binary format together with a modified SOAP Envelope, which are then
carried in separate parts of a MIME Multipart/Related [RFC
2387] package (see section 5.3.2 for details).

However, a SOAP message may travel over several hops between a sender and
the ultimate receiver, where each hop may be a different protocol binding. In
other words, a feature (e.g., message correlation, reliability etc.) that is
supported by the protocol binding in one hop may not be supported by another
along the message path. SOAP itself does not provide any mechanism for hiding
the differences in features provided by different underlying protocols.
However, any end-to-end or multi-hop feature that is required by a particular
application, but which may not be available in the underlying infrastructure
along the anticipated message path, can be compensated for by being
carried as a part of the SOAP message infoset, i.e., as a SOAP header block
specified in some module.

Thus it is apparent that there are a number of issues that have to be
tackled by an application designer to accomplish particular application
semantics, including how to take advantage of the native features of
underlying protocols that are available for use in the chosen environment. SOAP Part
1 section 4.2 provides a general framework for describing how SOAP-based
applications may choose to use the features provided by an underlying
protocol binding to accomplish particular application semantics. It is
intended to provide guidelines for writing interoperable protocol binding
specifications for exchanging SOAP messages.

Among other things, a binding specification must define one particular
feature, namely the message exchange pattern(s) that it supports. [SOAP Part2] defines two such message exchange patterns,
namely a
SOAP Request-Response message exchange pattern where one SOAP message is
exchanged in each direction between two adjacent SOAP nodes, and a SOAP
Response message exchange pattern which consists of a non-SOAP message
acting as a request followed by a SOAP message included as a part of the
response.

[SOAP Part2] also offers the application designer a
general feature called the
SOAP Web Method feature that allows applications full control over the
choice of the so-called "Web method" - one of GET, POST, PUT, DELETE whose
semantics are as defined in the [HTTP 1.1]
specifications - that may be used over the binding. This feature is defined
to ensure that applications using SOAP can do so in a manner which is
compatible with the architectural principles of the World Wide Web. (Very
briefly, the simplicity and scalability of the Web is largely due to the fact
that there are a few "generic" methods (GET, POST, PUT, DELETE) which can be
used to interact with any resource made available on the Web via a URI.) The
SOAP
Web Method feature is supported by the SOAP HTTP binding, although, in
principle, it is available to all SOAP underlying protocol bindings.

SOAP
Part 2 section 7 specifies one standardized
protocol binding using the binding framework of [SOAP
Part1], namely how SOAP is used in conjunction with HTTP as the
underlying protocol. SOAP Version 1.2 restricts itself to the definition of a
HTTP binding allowing only the use of the POST method in conjunction with the
Request-Response message exchange pattern and the GET method with the SOAP
Response message exchange pattern. Other specifications in future could
define SOAP bindings to HTTP or other transports that make use of the other
Web methods (i.e., PUT, DELETE).

The next sections show examples of two underlying protocol bindings for
SOAP, namely those to [HTTP 1.1] and email. It should be
emphasized again that the only normative binding for SOAP 1.2 messages is to
[HTTP 1.1]. The examples in section
4.2 showing email as a transport mechanism for SOAP is simply meant to
suggest that other choices for the transfer of SOAP messages are possible,
although not standardized at this time. A W3C Note [SOAP
Email Binding] offers an application of the SOAP protocol binding
framework of [SOAP Part1] by describing an experimental
binding of SOAP to email transport, specifically [RFC
2822]-based message transport. The discussion of [MTOM] and its concrete realization in an HTTP binding is
provided in section 5.3.

4.1 The SOAP HTTP Binding

HTTP has a well-known connection model and a message exchange pattern. The
client identifies the server via a URI, connects to it using the underlying
TCP/IP network, issues a HTTP request message and receives a HTTP response
message over the same TCP connection. HTTP implicitly correlates its request
message with its response message; therefore, an application using this
binding can chose to infer a correlation between a SOAP message sent in the
body of a HTTP request message and a SOAP message returned in the HTTP
response. Similarly, HTTP identifies the server endpoint via a URI, the Request-URI,
which can also serve as the identification of a SOAP node at the server.

HTTP allows for multiple intermediaries between the initial client and the
origin
server identified by the Request-URI, in which case the request/response
model is a series of such pairs. Note, however, that HTTP intermediaries are
distinct from SOAP intermediaries.

The HTTP binding in [SOAP
Part2] makes use of theSOAP
Web Method feature to allow applications to choose the so-called Web
method - restricting it to one of GET or POST - to use over the HTTP message
exchange. In addition, it makes use of two message exchange patterns that
offer applications two ways of exchanging SOAP messages via HTTP: 1) the use of the
HTTP POST method for conveying SOAP messages in the bodies of HTTP request
and response messages, and 2) the use of the HTTP GET
method in a HTTP request to return a SOAP message in the body of a HTTP
response. The first usage pattern is the HTTP-specific instantiation of a
binding feature called the SOAP request-response message exchange pattern, while the second uses a feature
called the SOAP response message exchange pattern.

The purpose of providing these two types of usages is
to accommodate the two interaction paradigms which are well established on
the World Wide Web. The first type of interaction allows for the use of data
within the body of a HTTP POST to create or modify the state of a resource
identified by the URI to which the HTTP request is destined. The second type
of interaction pattern offers the ability to use a HTTP GET request to obtain
a representation of a resource without altering its state in any way. In the
first case, the SOAP-specific aspect of concern is that the body of the HTTP
POST request is a SOAP message which has to be processed (per the SOAP
processing model) as a part of the application-specific processing required
to conform to the POST semantics. In the second case, the typical usage that
is forseen is the case where the representation of the resource that is being
requested is returned not as a HTML, or indeed a generic XML document, but as
a SOAP message. That is, the HTTP content type header of the response message
identifies it as being of media type "application/soap+xml"
[RFC 3902]. Presumably,
there will be publishers of resources on the Web who determine that such
resources are best retrieved and made available in the form of SOAP messages.
Note, however, that resources can, in general, be made available in multiple
representations, and the desired or preferred representation is indicated by
the requesting application using the HTTP Accept
header.

One further aspect of
the SOAP HTTP binding is the question of how an application determines which
of these two types of message exchange patterns to use.
[SOAP Part2] offers guidance on
circumstances when applications may use one of the two specified message
exchange patterns. (It is guidance - albeit a strong one - as it is phrased
in the form of a "SHOULD" in the
specifications rather than an absolute requirement identified by the word
"MUST", where these
words are interpreted as defined in the IETF [RFC
2119].) The SOAP response message exchange pattern with the HTTP GET
method is used when an application is assured that the message exchange is
for the purposes of information retrieval, where the information resource is
"untouched" as a result of the interaction. Such interactions are referred to
as safe and
idempotent in the HTTP specification. As the
HTTP SOAP GET usage does not allow for a SOAP message in the request,
applications that need features in the outbound interaction that can only be
supported by a binding-specific expression within the SOAP infoset (i.e., as
SOAP header blocks) obviously cannot make use of this message exchange
pattern. Note that the HTTP POST binding is available for use in all
cases.

The following
subsections provide examples of the use of these two message exchange
patterns defined for the HTTP binding.

4.1.1 SOAP HTTP GET Usage

Using the HTTP binding with the SOAP Response message exchange pattern is restricted to the HTTP GET method. This means that
the response to a HTTP GET request from a requesting SOAP node is a SOAP
message in the HTTP response.

Example 8a shows a HTTP GET directed by the
traveller's application (in the continuing travel reservation scenario) at
the URI
http://travelcompany.example.org/reservations?code=FT35ZBQ >where the traveler's itinerary may be viewed. (How
this URL was made available can be seen in
Example 5a.)

The HTTP Accept
header is used to indicate the preferred representation of the resource being
requested, which in this example is an "application/soap+xml" media type for
consumption by a machine client, rather than the "text/html" media type for
rendition by a browser client for consumption by a human.

Example 8b shows the HTTP
response to the GET in Example 8a. The body
of the HTTP response contains a SOAP message showing the travel details. A
discussion of the contents of the SOAP message is postponed until section 5.2 , as it is not relevant, at this point, to
understanding the HTTP GET binding usage.

Note that the
reservation details could well have been returned as an (X)HTML document, but
this example wanted to show a case where the reservation application is
returning the state of the resource (the reservation) in a data-centric media
form (a SOAP message) which can be machine processed, instead of (X)HTML
which would be processed by a browser. Indeed, in the most likely anticipated
uses of SOAP, the consuming application will not be a browser.

Also, as shown in the example, the use of
SOAP in the HTTP response body offers the possibility of expressing some
application-specific feature through the use of SOAP headers. By using SOAP,
the application is provided with a useful and consistent framework and
processing model for expressing such features.

4.1.2 SOAP HTTP POST Usage

Using the HTTP binding with the
SOAP Request-Response message exchange pattern is restricted to the HTTP
POST method. Note that the use of this message exchange pattern in the SOAP
HTTP binding is available to all applications, whether they involve the
exchange of general XML data or RPCs (as in the following examples)
encapsulated in SOAP messages.

Examples 9 and
10 show an example of a HTTP binding
using the SOAP Request-Response message exchange
pattern, using the same scenario as that
for Example 4 and Example 5a, respectively, namely conveying an RPC and its return
in the body of a SOAP message. The examples and discussion in this section
only concentrate on the HTTP headers and their role.

Example 9 shows an RPC request directed at
the travel service application. The SOAP message is sent in the body of a
HTTP POST method directed at the URI identifying the "Reservations" resource
on the server travelcompany.example.org. When using HTTP, the Request-URI
indicates the resource to which the invocation is "posted". Other than
requiring that it be a valid URI, SOAP places no formal restriction
on the form of the request URI (see [RFC 3986] for more
information on URIs). However, one of the principles of the Web architecture
is that all important resources be identified by URIs. This implies that most
well-architected SOAP services will be embodied as a large number of
resources, each with its own URI. Indeed, many such resources are likely to
be created dynamically during the operation of a service, such as, for
instance, the specific travel reservation shown in the example. So, a
well-architected travel service application should have different URIs for
each reservation, and SOAP requests to retrieve or manipulate those
reservations will be directed at their URIs, and not at a single monolithic
"Reservations" URI, as shown in Example 9. Example 13 in section 4.1.3
shows the preferred way to address resources such as a particular travel
reservation. Therefore, we defer until section 4.1.3
further discussion of Web architecture compatible SOAP/HTTP usage.

When placing SOAP messages in HTTP bodies, the HTTP Content-type header
must be chosen as "application/soap+xml" [RFC 3902]. (The
optional charset parameter, which can take the value of "utf-8" or "utf-16", is shown in this
example, but if it is absent the character set rules for freestanding [XML 1.0] apply to the body of the HTTP request.)

Example 10 shows the RPC return (with
details omitted) sent by the travel service application in the corresponding
HTTP response to the request from Example 5a. SOAP,
using HTTP transport, follows the semantics of the HTTP status codes for
communicating status information in HTTP. For example, the 2xx series of HTTP
status codes indicate that the client's request (including the SOAP
component) was successfully received, understood, and accepted etc.

If an error occurs processing the request, the HTTP binding specification
requires that a HTTP 500 "Internal Server Error" be used with an embedded
SOAP message containing a SOAP fault indicating the server-side processing
error.

4.1.3 Web Architecture Compatible SOAP Usage

One of the most central concepts of the World Wide Web is that of a URI as
a resource identifier. SOAP services that use the HTTP binding and wish to
interoperate with other Web software should use URIs to address all important
resources in their service. For example, a very important - indeed
predominant - use of the World Wide Web is pure information retrieval, where
the representation of an available resource, identified by a URI, is fetched
using a HTTP GET request without affecting the resource in any way. (This is
called a safe and
idempotent method in HTTP terminology.) The
key point is that the publisher of a resource makes available its URI, which
consumers may "GET".

There are many
instances when SOAP messages are designed for uses which are purely for
information retrieval, such as when the state of some resource (or object, in
programming terms) is requested, as opposed to uses that perform resource
manipulation. In such instances, the use of a SOAP body to carry the request
for the state, with an element of the body representing the object in
question, is seen as counter to the spirit of the Web because the resource is
not identified by the Request-URI of the HTTP GET. (In some SOAP/RPC
implementations, the HTTP Request-URI is often not the identifier of the
resource itself but some intermediate entity which has to evaluate the SOAP
message to identify the resource.)

To highlight the changes needed, Example 12a shows the way that is not
recommended for doing safe information retrieval on the Web. This is an
example of an RPC carried in a SOAP message, again using the travel
reservation theme, where the request is to retrieve the itinerary for a
particular reservation identified by one of the parameters,
reservationCode, of the RPC. (For purposes of this discussion,
it is assumed that the application using this RPC request does not need
features which require the use of SOAP headers.)

This representation is discouraged in cases where the operation is a "safe"
retrieval (i.e., it has no side effects)

Note that the resource to be retrieved is not
identified by the target URI in the HTTP request but has to be obtained by
looking within the SOAP envelope. Thus, it is not possible, as would be the
case with other "gettable" URIs on the Web, to make this available via HTTP
alone to consumers on the World Wide Web.

SOAP
Part 2 section 4.1 offers recommendations on how RPCs that constitute
safe and idempotent information retrievals may be defined in a Web-friendly
manner. It does so by distinguishing aspects of the method and specific
parameters in an RPC definition that serve to identify resources from those
that serve other purposes. In Example 12a, the
resource to be retrieved is identified by two things: the first is that it is
an itinerary (part of the method name), and the second is the reference to a
specific instance (a parameter to the method). In such a case, the
recommendation is that these resource-identifying parts be made available in
the HTTP Request-URI identifying the resource, as for example, as follows:
http://travelcompany.example.org/reservations/itinerary?reservationCode=FT35ZBQ.

Furthermore, when an RPC definition is such that
all parts of its method description can be described as resource-identifying,
the entire target of the RPC may be identified by a URI. In this case, if the
supplier of the resource can also assure that a retrieval request is safe,
then SOAP Version 1.2 recommends that the choice of the Web method property
of GET and the use of the SOAP
Response message exchange pattern be used as described in section 4.1.1. This will ensure that the SOAP RPC is
performed in a Web architecture compatible manner. Example 12b shows the preferred way for a SOAP
node to request the safe retrieval of a resource.

The Web architecture compatible alternative to representing the RPC in Example 12a

It should be noted that SOAP Version 1.2 does not
specify any algorithm on how to compute a URI from the definition of an RPC
which has been determined to represent pure information retrieval.

Note, however, that if
the application requires the use of features that can only have a
binding-specific expression within the SOAP infoset, i.e., using SOAP header
blocks, then the application must choose HTTP POST method with a SOAP message
in the request body.

It also requires the use of the SOAP Request-Response message exchange pattern implemented via a HTTP POST if the RPC description
includes data (parameters) which are not resource-identifying. Even in this
case, the HTTP POST with a SOAP message can be represented in a Web-friendly
manner. As with the use of the GET, [SOAP Part2]
recommends for the general case that any part of the SOAP message that serves
to identify the resource to which the request is POSTed be identified in the
HTTP Request-URI. The same parameters may, of course, be retained in the
SOAP env:Body element. (The parameters must be retained in the Body
in the case of a SOAP-based RPC as these are related to the procedure/method
description expected by the receiving application.)

Example 13 is the same as that in Example 9, except that the HTTP Request-URI has
been modified to include the reservation code, which serves to identify the
resource (the reservation in question, which is being confirmed and paid
for).

RPC from Example 4 carried in an HTTP POST Request in
a Web-friendly manner

In Example 13, the resource to be
manipulated is identified by two things: the first is that it is a
reservation (part of the method name), and the second is the specific
instance of a reservation (which is the value of the parameter
code to the method). The remainder of the parameters in the RPC
such as the creditCard number are not resource-identifying, but
are ancillary data to be processed by the resource. It is the recommendation
of [SOAP Part2] that resources that may be accessed by
SOAP-based RPCs should, where practical, place any such resource-identifying
information as a part of the URI identifying the target of the RPC. It
should be noted, however, that [SOAP Part2] does not
offer any algorithm to do so. Such algorithms may be developed in future.
Note, however, that all the resource-identifying elements have been retained
as in Example 9 in their encoded form in the
SOAP env:Body element.

In other words, as seen from the above
examples, the recommendation in the SOAP specifications is to use URIs in a
Web-architecture compatible way - that is, as resource identifiers - whether
or not it is GET or POST that is used.

4.2 SOAP Over Email

Application developers can use the Internet email infrastructure to move
SOAP messages as either email text or attachments. The examples shown below
offer one way to carry SOAP messages, and should not be construed as being
the standard way of doing so. The SOAP Version 1.2 specifications do not
specify such a binding. However, there is a non-normative W3C Note
[SOAP Email Binding] describing an email binding for
SOAP, its main purpose being to demonstrate the application of the general
SOAP Protocol Binding Framework described in [SOAP Part
1].

Example 14 shows the travel reservation request
message from Example 1 carried as an email message
between a sending and receiving mail user agent. It is implied that the
receiver node has SOAP capabilities, to which the body of the email is
delivered for processing. (It is assumed that the sending node also has SOAP
capabilities so as to be able to process any SOAP faults received in
response, or to correlate any SOAP messages received in response to this
one.)

Although an email is a one-way message exchange, and no guarantee of
delivery is provided, email infrastructures like the Simple Mail Transport
Protocol (SMTP) specification [SMTP] offer a delivery
notification mechanism which, in the case of SMTP, are called Delivery Status
Notification (DSN) and Message Disposition Notification (MDN). These
notifications take the form of email messages sent to the email address
specified in the mail header. Applications, as well as email end users, can
use these mechanisms to provide the status of an email transmission, but
these, if delivered, are notifications at the SMTP level. The application
developer must fully understand the capabilities and limitations of these
delivery notifications or risk assuming a successful data delivery when none
occurred.

SMTP delivery status messages are separate from message processing at the
SOAP layer. Resulting SOAP responses to the contained SOAP data will be
returned through a new email message which may or may not have a link to the
original requesting email at the SMTP level. The use of the [RFC 2822] In-reply-to: header can achieve a
correlation at the SMTP level, but does not necessarily offer a correlation
at the SOAP level.

Example 15 is exactly the same scenario as
described for Example 2, which shows the SOAP message
(body details omitted for brevity) sent from the travel service application
to the travel reservation application seeking clarification on some
reservation details, except that it is carried as an email message. In this
example, the original email's Message-Id is carried in the
additional email header In-reply-to:, which correlates email
messages at the SMTP level, but cannot provide a SOAP-specific correlation.
In this example, the application relies on the reservation
header block to correlate SOAP messages. Again, how such correlation is
achieved is application-specific, and is not within the scope of SOAP.

SOAP message from Example 2 carried in an email
message with a header correlating it to a previous message.

5. Advanced Usage Scenarios

5.1 Using SOAP Intermediaries

The travel reservation scenario used throughout the primer offers an
opportunity to expose some uses of SOAP intermediaries. Recall that the basic
exchange was the exchange of a travel reservation request between a travel
reservation application and a travel service application. SOAP does not
specify how such a message path is determined and followed. That is outside
the scope of the SOAP specification. It does describe, though, how a SOAP
node should behave if it receives a SOAP message for which it is not the
ultimate receiver. SOAP Version 1.2 describes two types of intermediaries: forwarding
intermediaries and active
intermediaries.

A forwarding
intermediary is a SOAP node which, based on the semantics of a header
block in a received SOAP message or based on the message exchange pattern in
use, forwards the SOAP message to another SOAP node. For example, processing
a "routing" header block describing a message path feature in an incoming
SOAP message may dictate that the SOAP message be forwarded to another SOAP
node identified by data in that header block. The format of the SOAP header
of the outbound SOAP message, i.e., the placement of inserted or reinserted
header blocks, is determined by the overall processing at this
forwarding intermediary based on the semantics of the processed
header blocks.

An active
intermediary is one that does additional processing on an incoming SOAP
message before forwarding the message using criteria that are not described
by incoming SOAP header blocks, or by the message exchange pattern in use.
Some examples of such active intervention at a SOAP node could be, for
instance, encrypting some parts of a SOAP message and providing the
information on the cipher key in a header block, or including some additional
information in a new header block in the outbound message providing a
timestamp or an annotation, for example, for interpretation by appropriately
targeted nodes downstream.

One mechanism by which an active intermediary can describe the
modifications performed on a message is by inserting header blocks into the
outbound SOAP message. These header blocks can inform downstream SOAP nodes
acting in roles whose correct operation depends on receiving such
notification. In this case, the semantics of such inserted header blocks
should also call for either the same or other header blocks to be
(re)inserted at subsequent intermediaries as necessary to ensure that the
message can be safely processed by nodes yet further downstream. For example,
if a message with header blocks removed for encryption passes through a
second intermediary (without the original header blocks being decrypted and
reconstructed), then indication that the encryption has occurred must be
retained in the second relayed message.

In the following example, a SOAP node is introduced in the message path
between the travel reservation and travel service applications, which
intercepts the message shown in Example 1. An example
of such a SOAP node is one which logs all travel requests for off-line review
by a corporate travel office. Note that the header blocks
reservation and passenger in that example are
intended for the node(s) that assume the role "next", which means that it is
targeted at the next SOAP node in the message path that receives the message.
The header blocks are mandatory (the mustUnderstand attribute is
set to "true"), which means that the node must have knowledge (through an
external specification of the header blocks' semantics) of what to do. A
logging specification for such header blocks might simply require that
various details of the message be recorded at every node that receives such a
message, and that the message be relayed along the message path unchanged.
(Note that the specifications of the header blocks must require that the same
header blocks be reinserted in the outbound message, because otherwise, the
SOAP processing model would require that they be removed.) In this case, the
SOAP node acts as a forwarding intermediary.

A more complex scenario is one where the received SOAP message is amended
in some way not anticipated by the initial sender. In the following example,
it is assumed that a corporate travel application at the SOAP intermediary
attaches a header block to the SOAP message from Example
1 before relaying it along its message path towards the travel service
application - the ultimate recipient. The header block contains the
constraints imposed by a travel policy for this requested trip. The
specification of such a header block might require that the ultimate
recipient (and only the ultimate recipient, as implied by the absence of the
role attribute) make use of the information conveyed by it when
processing the body of the message.

Example 16 shows an active intermediary inserting
an additional header block, travelPolicy, intended for the
ultimate recipient which includes information that qualifies the
application-level processing of this travel request.

SOAP message from Example 1 for a travel reservation
after an active intermediary has inserted a mandatory header intended for the
ultimate recipient of the message

5.2 Using Other Encoding Schemes

Even though SOAP Version 1.2 defines a particular encoding scheme (see SOAP
Part 2 section 3), its use is optional and the specification makes clear
that other encoding schemes may be used for application-specific data within
a SOAP message. For this purpose it provides the attributeenv:encodingStyle, of type xs:anyURI, to
qualify header blocks, any child elements of the SOAP env:Body,
and any child elements of the env:Detail element and their
descendants. It signals a serialization scheme for the nested contents, or at
least the one in place until another element is encountered which indicates
another encoding style for its nested contents. The choice of the value for
the env:encodingStyle attribute is an application-specific
decision and the ability to interoperate is assumed to have been settled
"out-of-band". If this attribute is not present, then no claims are being
made about the encoding being used.

The use of an alternative encoding scheme is illustrated in Example 17. Continuing with the travel reservation
theme, this example shows a SOAP message which is sent to the passenger from
the travel service after the reservation is confirmed, showing the travel
details. (The same message was used in Example 8b in another context.)

SOAP message showing the use of an alternative encoding for the
Body element

In Example 17, the body of the SOAP message
contains a description of the itinerary using the encoding of a graph of
resources and their properties using the syntax of the Resource Description
Framework (RDF) [RDF]. (Very briefly, as RDF syntax or
usage is not the subject of this primer, an RDF graph relates resources -
such as the travel reservation resource available at
http://travelcompany.example.org/reservations?code=FT35ZBQ - to
other resources (or values) via properties, such as the
passenger, the outbound and return
dates of travel. The RDF encoding for the itinerary might have been chosen,
for example, to allow the passenger's travel application to store it in an
RDF-capable calendar application, which could then be queried in complex
ways.)

5.3 Optimized serialization of SOAP messages

There are many use cases where it is necessary for an application to send a
large amount of binary (i.e., non-textual) data (e.g., a JPEG image, binary
executable etc.) in a SOAP message. The conventional way of conveying binary
data in an XML document, such as the SOAP env:Envelope, is to
transform the binary data into a character representation of type xs:base64Binary
using the Base64 content-transfer-encoding scheme defined in [RFC 2045]. The disadvantages of this approach are that
there is a significant increase in message size, as well as a potential
processing overhead in encoding/decoding the binary data to/from its
character representation, which may create throughput problems in case of
message transmission over low bandwidth links or SOAP nodes with low
processing power.

The [MTOM] specification provides a mechanism to
support such use cases. It should be noted, though, that the specification
does not address the general problem of handling the inclusion of non-XML
content in arbitrary XML documents, but confines itself to the specific case
of SOAP message transmission optimization for certain type of content.

In order to allow for independence from the underlying protocol binding,
so that the optimization mechanism may be available over a variety of
transports, as well as to retain the principal SOAP binding paradigm - that
the SOAP message infoset, however serialized, be transmitted unchanged
between adjacent nodes - [MTOM] defines an Abstract
SOAP Transmission Optimization feature, of which one implementation is provided for the particular case of
HTTP-based transmission of an optimized SOAP message in a MIME
Multipart/Related [RFC 2387] package. This makes use of
the [XOP] format (on which more in Section 5.3.2) which is an alternative serialization of an
XML infoset geared towards a more eficient processing and representation of
Base64-encoded content.

5.3.1 The Abstract SOAP Transmission Optimization Feature

The Abstract
SOAP Transmission Optimization feature is defined for certain element
information item in the SOAP message infoset which are identified as
potential candidates for optimization. XML infosets identify the structure
and content of XML documents, but not the data type of the contents of
elements and attributes. One way to identify these would require schema
validation of the infoset, something which is not a
requirement for SOAP. A more likely possibility is that the sending
application already "knows" the type of data - a binary stream, and perhaps
also the nature of the media type that it represents - that it wishes to
transmit because that is the way in which the data is already available to
it. The Abstract
SOAP Transmission Optimization feature assumes that the type information
for those element information items which are potential candidates
for optimization are somehow available to the sender of a SOAP message. This
feature is restricted to the optimization of character information items of
any element information item in the SOAP message infoset which is known to be
of type xs:base64Binary
in its canonical lexical form (see [Schema Part2] Section
3.2.16 base64Binary). (The rationale for the restriction to the canonical
form of xs:base64Binary is provided at the
end of this section.)

To motivate the need for [MTOM], consider the example
of a SOAP message sent in response to the request for the travel itinerary in
Example 12b. The travel reservation
application may wish to send, in addition to the information which can
readily be represented in XML, a corporate logo, a map of the destination and
other such information which is available in binary format (e.g., image
files). If there were only a small amount of non-XML data, it may be possible
to convert such data to its base64 encoding and convey the result in a SOAP
message sent in the HTTP response as shown in Example
18 (with irrelevant content indicated by ellipses for brevity, and line
breaks added for clarity).

Example 18 highlights two elements contained
within the SOAP env:Body>, namely
o:travelAgencyLogo and r:areaMap, containing the
base64 encoding of binary data corresponding to a corporate logo and an area
map. While small amounts of binary data can be placed in a SOAP message using
the base64 encoding without incurring the performance overheads noted
earlier, binary data anticipated in typical use cases is typically quite
large, often many orders of magnitude larger than the XML content. To avoid
the performance penalty in such circumstances, [MTOM]
offers an optimization that avoids the need to base64-encode large binary
content. (Note that SOAP nodes that do not implement [MTOM] have no choice but to carry binary data re-encoded in
its base64 character representation, as in Example
18.)

The Abstract
SOAP Transmission Optimization feature provides the optimization by
conceptually describing the binary data that needs to be conveyed as the
content of an element information item in the SOAP message infoset in terms
of its base64 encoding. While this character based representation is
conceptually present at the sender and receiver, [MTOM] works on the
optimistic assumption that the sending and receiving applications will not
actually need the character-based representation of the binary value, and
therefore there will be no real processing overhead in conversion between the
binary value and its base64 encoding. Similarly, the implementation of this
feature using the [XOP] format (more details are provided in section 5.3.2) employs a MIME Multipart/Related [RFC 2387] package to convey the binary data as an
"attachment" referenced from within a modified, serialized SOAP message;
therefore, there is also no overhead of increased message size.

As noted earlier, it is assumed that the sending implementation somehow
knows or determines the type information of the element information items
that are candidates for potential optimization; otherwise the optimization
feature does not work. The scope of [MTOM] is solely to
optimize the transmission of the SOAP message infoset for those element
information items that have base64 encoded binary data in canonical form as
their content.

As with all features, [MTOM] needs a SOAP protocol
binding to transfer the optimized serialization. Recall from Section 4 that a SOAP protocol binding must transfer
the SOAP message infoset in such a way that it can be reconstructed at the
receiver unchanged. An MTOM-aware binding is one where a sender can serialize
a SOAP message infoset by transmitting the actual value - that is, the actual
bits - of certain element information items known to be in the canonical
lexical representation of type xs:base64Binary rather than their
lexical character representation. A receiver supporting this binding can,
from the received value, reconstruct, at least conceptually, the lexical
character representation if that is required by the application.

[MTOM] provides an enhancement to the existing SOAP HTTP binding to
provide an implementation of the Abstract
SOAP Transmission Optimization feature. It uses the [XOP]-based inclusion mechanism described in section 5.3.2,
and places the resulting MIME Multipart/Related package in the body of a HTTP
message.

As noted earlier, applications, in many implementations, will deal
directly with the binary values and there is no implication that a base64
encoded character representation of the received value needs to be created,
if there is no need to do so. However, there are instances when there may be
a need to obtain the character representation, for example at a SOAP
intermediary which has to forward the message on a non-MTOM-aware binding.

One important subtlety in ensuring
that the original message infoset can be reconstructed faithfully is to
mandate, as does [MTOM], that the original base64
encoded characters be in their canonical form. [XML Schema
Part2] allows for multiple lexical representations of the
xs:base64Binary data type, mainly in the handling of white
space, and therefore defines a canonical form which permits a 1-to-1
correspondence between a binary value and its lexical representation. By
restricting itself to optimization candidates which are in the canonical form
of xs:base64Binary, it can be ensured that the transferred
message infoset is reproduced unchanged.

Therefore, in the following sections, whenever we, for the sake of
brevity, refer to base64-encoded data, the reader should keep in mind that we
mean XML element content whose type is in the canonical lexical
representation of xs:base64Binary.

5.3.2 The Optimized Transmission Serialization Format

The next step in implementing the Abstract
SOAP Transmission Optimization feature is to define the format in which
the SOAP message infoset (with potential optimization candidates identified,
as described in the previous section) is serialized in an optimal way for
transmission. The serialization technique is described in [MTOM] by making use of an "inclusion" technique specified
in the XML-binary Optimized Packaging [XOP]
specification together with a MIME Multipart/Related packaging ([RFC
2387]).

[XOP] defines an xop:Include
element that ties, at a SOAP binding level, the binary content for an element
to its infoset representation as base64-encoded character information items
in the [children] property of that element information item. A SOAP binding
that is capable of optimized serialization of an infoset containing such
binary data represented by their character information items uses this
xop:Include
element in the SOAP envelope as a placeholder to link (using an
href attribute) to the optimized (i.e., binary) data carried
along with the SOAP envelope in an overall package.

The overall package chosen is the extensible MIME Multipart/Related [RFC 2387] format. The root body part of this MIME package
contains the XML 1.0 serialization of the SOAP env:Envelope,
modified by the presence of one (or more) xop:Include
element(s), while the other (related) body part(s) of the MIME package
contain the compact (i.e, binary) data referenced by the
xop:Include element(s).

The serialization of the SOAP message from Example
18, converted to this optimized format using [XOP], is shown in Example 19a.

In Example 19a, the conventional MIME
Multipart/Related package conveys a compound "object" broken up into multiple
inter-related body parts. The "start" parameter of the overall Content-Type
conveys, via a Content-ID, the body part which contains the compound object's
"root", while the media type parameter value of "application/xop+xml"
identifies the contents as an XML document serialized using the [XOP] format.
The "startinfo" parameter of the package shows that this root part is the XML
1.0 serialization of the SOAP env:Envelope modified by the
inclusion of xop:Include elements where appropriate.

Compared with Example 18, note, in Example 19a, the presence of the two xop:Include
elements which replace the character representations of the binary data
corresponding to the company logo and the lodging area map. Each of these
elements provides via the href attribute the link by which the
binding knows which MIME body part contains the binary data that corresponds
to the (canonical form of the) equivalent base64-encoded character
representation.

When such an optimized MIME Multipart/Related package based on the [XOP]
format is sent in a HTTP message, [MTOM] Section
4.3 requires that the resultant MIME headers are sent as HTTP headers,
while the remainder of the package is placed in the HTTP body. Example 19b shows the SOAP message from Example 19a returned in a HTTP response (with the
relevant HTTP headers highlighted).

In Example 19b,
the MIME Multipart/Related headers arising from the [XOP] format (see Example 19a) are carried as HTTP headers in the HTTP
200 OK response.

5.3.3 Using the Resource Representation SOAP Header Block

Another optimization that has been identified as useful for processing a
SOAP message which includes URI-based references to Web resources is one
where the sender includes a representation of each such resource in
the SOAP message to either the ultimate receiver or an intermediary. This
helps in situations where the processing of the SOAP message depends on
dereferencing the URIs, but which may not be possible because the receiver is
not able or wishes to avoid the overhead of the network traffic needed to do
so. The gain is even greater if the same resource (the image of a logo, say)
is referenced multiple times within the message.

The Resource Representation SOAP Header Block [ResRep] specification describes a SOAP header block,
containing a rep:Representation
element, which defines how URI-based representations of resources referenced
within a SOAP message infoset may be carried and processed by an identified
receiver. Its use is illustrated by examples that follow.

Recall, from Example 18, that a base64-encoded
form of the travel agency logo was sent in the SOAP message. However, this
may well have been included by providing a HTTP URL link to the location from
which the (ultimate) receiver could retrieve the image as a part of
processing the message. This is shown, with all inessentials deleted, in Example 20.

Sample SOAP message based on Example 18 showing the
travel agency logo element content as a link to a Web resource

In Example 20, the expectation is that the
contents of the o:image element would be obtained by
dereferencing the URL identified by the o:source attribute.
However, as identified earlier, if a situation were anticipated where the
processing overhead of dereferencing the URI were considered unacceptable, a
representation of the logo image can be sent using the rep:Representation
element, as shown in Example 21 (with the header
highlighted).

If the binary content representing the resource were available to the
sender, and sending the base64-encoded form of that (presumably large) binary
content was deemed inefficient, the use of the
rep:Representation element can be combined with [MTOM] and the [XOP]
format to gain the efficiencies of that feature. This is shown in Example 22, with the xop:Include element
highlighted.

Sample SOAP message based on Example 21 showing the
representation of a Web resource (an image of the travel agency logo) carried
in the Representation header using the MTOM/XOP optimization.

6. Changes Between SOAP 1.1 and SOAP 1.2

SOAP Version 1.2 has a number of changes in syntax and provides additional
(or clarified) semantics from those described in [SOAP
1.1]. The following is a list of features where the two specifications
differ. The purpose of this list is to provide the reader with a quick and
easily accessible summary of the differences between the two specifications.
The features have been put in categories purely for ease of reference, and in
some cases, an item might equally well have been placed in another
category.

Document structure

The SOAP 1.2 specifications have been provided in two parts. [SOAP Part1] provides an abstract Infoset-based
definition of the SOAP message structure, a processing model and an
underlying protocol binding framework, while [SOAP
Part2] provides serialization rules for conveying that infoset as
well as a particular HTTP binding.

SOAP 1.2 will not spell out the acronym.

SOAP 1.2 has been rewritten in terms of XML infosets, and not as
serializations of the form <?xml....?> required by SOAP 1.1.

Additional or changed syntax

SOAP 1.2 does not permit any element after the body. The SOAP 1.1 schema definition
allowed for such a possibility, but the textual description is silent
about it.

SOAP 1.2 does not allow the env:encodingStyle attribute to
appear on the SOAP env:Envelope, whereas SOAP 1.1 allows it
to appear on any element. SOAP 1.2 specifies specific elements where this
attribute may be used.

SOAP 1.2 defines the new env:NotUnderstood header element
for conveying information on a mandatory header block which could not be
processed, as indicated by the presence of an
env:MustUnderstand fault code. SOAP 1.1 provided the fault
code, but no details on its use.

In the SOAP 1.2 infoset-based description, the
env:mustUnderstand attribute in header elements takes the
(logical) value "true" or "false", whereas in SOAP 1.1 they are the
literal value "1" or "0" respectively.

SOAP 1.2 provides a new fault code
DataEncodingUnknown.

The various namespaces defined by the two protocols are of course
different.

SOAP 1.2 replaces the attribute env:actor with
env:role but with essentially the same semantics.

SOAP 1.2 defines a new attribute, env:relay, for header
blocks to indicate if unprocessed header blocks should be forwarded.

SOAP 1.2 defines two new roles, "none" and "ultimateReceiver", together
with a more detailed processing model on how these behave.

SOAP 1.2 has removed the "dot" notation for fault codes, which are now
simply an XML Qualified Name, where the namespace prefix is the SOAP
envelope namespace.

SOAP 1.2 uses the element names env:Code and
env:Reason, respectively, for what used to be called
faultcode and faultstring in SOAP 1.1. SOAP 1.2
also allows multiple env:Text child elements of
env:Reason qualified by xml:lang to allow
multiple language versions of the fault reason.

SOAP 1.2 provides a hierarchical structure for the mandatory SOAP
env:Code sub-element in the env:Fault element,
and introduces two new optional subelements, env:Node and
env:Role.

SOAP 1.2 removes the distinction that was present in SOAP 1.1 between
header and body faults as indicated by the presence of the
env:Details element in env:Fault. In SOAP 1.2,
the presence of the env:Details element has no significance
as to which part of the fault SOAP message was processed.

In the SOAP 1.2 HTTP binding, the SOAPAction HTTP header
defined in SOAP 1.1 has been removed, and a new HTTP status code 427 has
been sought from IANA for indicating (at the discretion of the HTTP
origin server) that its presence is required by the server application.
The contents of the former SOAPAction HTTP header are now
expressed as a value of an (optional) "action"
parameter of the "application/soap+xml" media type that is signaled in
the HTTP binding.

In the SOAP 1.2 HTTP binding, the Content-type header should be
"application/soap+xml" instead of "text/xml" as in SOAP 1.1. The IETF
registration for this new media type is [RFC 3902].

SOAP 1.2 provides a finer grained description of use of the various
2xx, 3xx, 4xx HTTP status codes.

Support of the HTTP extensions framework has been removed from SOAP
1.2.

SOAP 1.2 provides an additional message
exchange pattern which may be used as a part of the HTTP binding that
allows the use of HTTP GET for safe and idempotent information
retrievals.

SOAP 1.2 offers guidance on a Web-friendly
approach to defining RPCs where the procedure's purpose is purely "safe"
informational retrieval.

SOAP encodings

An abstract data model based on a directed edge labeled graph has been
formulated for SOAP 1.2. The SOAP 1.2 encodings are dependent on this
data model. The SOAP RPC conventions are dependent on this data model,
but have no dependencies on the SOAP encoding. Support of the SOAP 1.2
encodings and SOAP 1.2 RPC conventions are optional.

The syntax for the serialization of an array has been changed in SOAP
1.2 from that in SOAP 1.1.

The support provided in SOAP 1.1 for partially transmitted and sparse
arrays is not available in SOAP 1.2.

SOAP 1.2 has added an optional attribute enc:nodeType to
elements encoded using SOAP encoding that identifies its structure (i.e.,
a simple value, a struct or an array).

SOAP Part
1 Appendix A provides version management rules for a SOAP node that can
support the version transition from [SOAP 1.1] to SOAP
Version 1.2. In particular, in defines an env:Upgrade
header block which can be used by a SOAP 1.2 node on receipt of a [SOAP 1.1] message to send a SOAP fault message to the
originator to signal which version of SOAP it supports.

Namespaces in
XML (Second Edition), Tim Bray, Dave Hollander,
Andrew Layman, and Richard Tobin, Editors.
World Wide Web Consortium, 16 August 2006.
This version is http://www.w3.org/TR/2006/REC-xml-names-20060816.
The latest version is
available at http://www.w3.org/TR/REC-xml-names.

[XML Schema Part 1]

XML Schema Part 1:
Structures Second Edition, David Beech, Murray Maloney,
Henry S. Thompson, and Noah Mendelsohn, Editors.
World Wide Web Consortium, 28 October 2004.
This version is http://www.w3.org/TR/2004/REC-xmlschema-1-20041028/
The latest version is
available at http://www.w3.org/TR/xmlschema-1/.

RDF
Primer, Frank Manola and Eric Miller, Editors.
World Wide Web Consortium, 10 February 2004.
This version is http://www.w3.org/TR/2004/REC-rdf-primer-20040210/.
The latest version is
available at http://www.w3.org/TR/rdf-primer/.

XML
Information Set (Second Edition), Richard Tobin and
John Cowan, Editors.
World Wide Web Consortium, 04 February 2004.
This version is http://www.w3.org/TR/2004/REC-xml-infoset-20040204.
The latest version is
available at http://www.w3.org/TR/xml-infoset.

SOAP Version 1.2 Email Binding,
Highland Mary Mountain, Jacek Kopecky, Stuart Williams,
et. al., Editors.
World Wide Web Consortium, 03 July 2002.
This version is http://www.w3.org/TR/2002/NOTE-soap12-email-20020703.
The latest version
is available at http://www.w3.org/TR/soap12-email.

[XML Base]

XML
Base,
Jonathan Marsh, Editor.
World Wide Web Consortium, 27 June 2001.
This version is http://www.w3.org/TR/2001/REC-xmlbase-20010627/.
The latest version is
available at http://www.w3.org/TR/xmlbase/.

XML-binary Optimized
Packaging, Mark Nottingham, Noah Mendelsohn, Martin Gudgin,
and Hervé Ruellan, Editors.
World Wide Web Consortium,
25 January 2005.
This version is http://www.w3.org/TR/2005/REC-xop10-20050125/.
The latest version is
available at http://www.w3.org/TR/xop10/.

[ResRep]

Resource
Representation SOAP Header Block, Martin Gudgin,
Yves Lafon, and Anish Karmarkar, Editors.
World Wide Web Consortium,
25 January 2005.
This version is http://www.w3.org/TR/2005/REC-soap12-rep-20050125/.
The latest version is
available at http://www.w3.org/TR/soap12-rep/.

A. Acknowledgements (Non-Normative)

Highland Mary Mountain provided the initial material for the section on
the SMTP binding. Paul Denning provided material for a usage scenario, which
has since been moved to the SOAP Version 1.2 Usage Scenarios Working Draft.
Stuart Williams, Oisin Hurley, Chris Ferris, Lynne Thompson, John Ibbotson,
Marc Hadley, Yin-Leng Husband and Jean-Jacques Moreau provided detailed
comments on earlier versions of this document, as did many others during the
Last Call Working Draft review. Jacek Kopecky provided a list of RPC and SOAP
encoding changes.

Martin Gudgin reviewed the additional material in the second edition and
provided many helpful comments.